Germane gas is used in the semiconductor industry. It allows the application of strained silicon to computer CPUs and has become a key material in the newly emerging germanium channels and gates. Also, the germane gas is used in the formation of the intermediate SiGe layer of the triple junction of the 5th-generation amorphous silicon thin-film solar cell (a-Si solar cell), as it enhances absorption of light in the mid-wavelength range of 300-900 nm, and thus, improving the cell efficiency. Accordingly, with the expected increase in the demand on the next-generation thin-film solar cells, the demand on germane gas is also expected to grow rapidly.
Synthesis of germane gas and chemical reactions involved therein has been studied by many chemists since 1930s. Typical examples include a chemical method of reducing germanium dioxide (GeO2) or germanium tetrachloride (GeCl4) using sodium borohydride (NaBH4), lithium aluminum hydride (LiAlH4), etc., an electrochemical method of electrolyzing germanium dioxide and a high-energy method of reacting germanium (Ge) directly with hydrogen.
As for the existing methods of preparing germane gas using germanium dioxide or germanium tetrachloride, the yield is only about 70%. In particular, when monogermane gas is prepared using germanium dioxide which is easier to handle than germanium tetrachloride, it is difficult to prepare monogermane gas in high yield.
In this regard, U.S. Pat. No. 4,668,502 discloses that the yield of germane gas can be increased up to 97%, even when germanium dioxide (GeO2) is used as the same raw material, by varying reaction conditions, i.e. combinations of germanium dioxide concentration, alkali/germanium dioxide ratio in an aqueous alkali solution wherein germanium dioxide is dissolved, amount of alkali metal borohydride, acid concentration, reaction temperature, etc. Indeed, a high germane yield of around 90% was achieved when experiment was conducted according to the reaction conditions specified in the examples and claims of U.S. Pat. No. 4,668,502.
However, if the aqueous alkali solution wherein germanium dioxide and the alkali metal borohydride are dissolved is reacted with an aqueous acid solution in batches or continuously according to the above method, germane gas is produced explosively in short time and high reaction heat is accompanied. This means that if the preparation of germane gas is carried out in industrial scale, not in laboratory scale, it will be difficult to control its reaction rate and reaction heat. If the high reaction heat is not controlled adequately, its reaction temperature may increase rapidly (about 50° C. or higher) and higher germane may be formed, thereby negatively affecting the yield of monogermane gas.
The inventors of the present disclosure have made consistent efforts to solve the above problem. As a result, they have found out that the problem can be solved by mixing starting materials in a short time and removing reaction heat at the same time using a reactor having a microstructured channel.